This fret eraser was a test buy for me. I've been using Micro Mesh pads for a great shine on frets but they don't last forever and I wanted to see how the fret erasers do. That they should last a long time and give consistent results throughout their useable life seems like quite an advantage.
First off, this 100grit, does not give a mirror shine. It's roughly equivalent to 3200 grit micro mesh. That gets frets clean quickly and smooth enough to play very nicely but it's obviously not as good as finish as continuing with finer grits. If Thomann offered 2000 and 3000 grits, I'd definitely give them a try.
I think this a lot of work for the result. With 3" x 4" micro mesh pads I can work many frets at a time and the micro mesh conforms to the shape of the fret better. The fret erasers require more diligent work across the fret and while they do a great job quickly, you end up doing one fret at at time.
tl;dr I'm a little disappointed and finer grits are definitely required.